1. Field of the Invention
This invention relates to an improved lithium-sulfur dioxide electrochemical cell. More particularly, it relates to the use of iodine and iodide ion as electrolyte components in lithium-sulfur dioxide cells.
2. Description of the Prior Art.
A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals above hydrogen in the electromotive series of elements which are unstable in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water. Lithium has been of particular interest as an active metal for such high energy density cells since it is the most active of the metals in the electromotive series and has the ability in an electrochemical cell to provide the highest performance in watt-hours per kilogram of all known active metals.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as nickel or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell.
Lithium-sulfur dioxide electrochemical cells, particularly those which utilize a carbon cathode, are frequently characterized by a limited cathode discharge-rate capability. This occasionally results in an explosion when such carbon-cathode cells are subjected to forced discharge conditions in a battery pack. Such explosions are believed to be a result of cathode shutoff as a result of the plugging of pores in the cathode by discharge products, followed by a deposition of lithium at the cathode which results in a potentially explosive combination of lithium, carbon and discharge products. Unfortunately, this explosion hazard serves to limit the utility of such cells under conditions where forced discharge may occur.
Canadian Pat. No. 878,713, issued on Aug. 17, 1971, discloses an electrochemical cell which contains a aqueous electrolyte comprised of an organic solvent selected from the group consisting of alkylene carbonates and lactones and an ion-permeable barrier to separate the anode and cathode depolarizer. However, this patent contains no suggestion that sulfur dioxide could be used as a component of such a cell.
U.S. Pat. No. 4,145,484, issued to Goodson et al. on Mar. 20, 1979, is directed to an electrochemical cell which utilizes a lithium anode in combination with a solution of a quaternary ammonimum salt in liquid bromine. The bromine functions as the cathode depolarizer and it is disclosed that the bromine solution can be placed in direct contact with the lithium anode as a consequence of the formation of a film of lithium bromide on the and surface. This patent contains no suggestion that either iodine or sulfur dioxide could be utilized in an electrochemical cell.
U.K. Pat. No. 2,056,752, published June 2, 1983, discloses an electrochemical cell which contains an active metal anode in direct contact with an electrolyte which comprises a free halogen dissolved in a nonaqueous solvent. It is disclosed that lithium can be used as the anode, iodine may be used as the halogen and the nonaqueous solvent may be selected from a plethora of organic and inorganic materials which include tetrahydrofuran. It is further disclosed that the ionic conduction of the electrolyte can be facilitated by dissolving in it a metal salt, such as a lithium halide. However, this patent contains no mention of sulfur dioxide.
The above-mentioned U.S. Pat. No. 3,567,515 (Maricle et al.) discloses an electrochemical cell which contains a lithium anode and a nonaqueous conductive liquid electrolyte which comprises sulfur dioxide. This patent also discloses that the electrolyte may contain an organic solvent and a plethora of electrolyte salts which include lithium perchlorate, lithium halides, and tetra(loweralkyl)ammonium salts of halogens. U.S. Pat. No. 3,578,500 issued to Maricle et al. on May 11, 1971, contains a similar disclosure. However, these patents fail to suggest the use of iodine as an electrolyte component and require that the electrolyte salts be substantially inert with respect to chemical oxidation by sulfur dioxide.
It has been reported that iodide ion can be oxidized by sulfur dioxide, but only in the presence of oxygen (see Non-Aqueous Solvent Systems, T. C. Waddington, Ed., Academic Press, 1965, p. 266; and Progress in Physical Organic Chemistry, Vol. 1, S. G. Cohen et al., Ed., Interscience Publishers, 1963, p. 98). However, there has been no suggestion in the prior art that the oxidation of iodide ion by sulfur dioxide could be advantageously utilized for any purpose in an electrochemical cell.